The Most Extreme Migration on Earth?

Way up in the Himalayas, where thin air and low oxygen pressure hinder speech and movement, weary mountaineers have observed bar-headed geese (Anser indicus) honking away as they ascend powerfully overhead. Every year the geese make an epic migration from sea level in India up over the immense mountain range to their summer breeding grounds in Central Asia. A new study shows that they do it quickly and under their own steam, without the help of upslope tailwinds thought to loft them over the sky-high peaks. The discovery, argues the study’s lead author, makes the birds’ annual journey “the most extreme migration on Earth.”

In recent years, researchers have illuminated a number of adaptations that enable the bar-headed goose’s migratory feat. These include proportionally bigger lungs than many other bird species, a better supply of oxygen to the muscles and heart, denser capillaries, and hemoglobin that carries more oxygen. Even so, most experts have thought the birds couldn’t make the trip without the help of winds that rush predictably up the mountains during the day.

To settle the matter, biologist Lucy Hawkes, a postdoctoral researcher at Bangor University in the United Kingdom, and her colleague physiologist Charles Bishop, also of Bangor University, led an international team of 13 scientists in tracking the birds’ Himalayan overflights for the first time. The researchers captured 25 bar-headed geese in India, attached satellite tags to them that logged their location, altitude, and speed, and then freed them to make their springtime northbound migration. They also captured and tagged a total of 38 geese in Mongolia before their autumn southbound trip. In the end, the study included a smaller subset of birds whose tags successfully collected data each hour, as planned.

The northbound geese typically made the trip from sea level over mountain passes of up to 6000 meters in just 7 or 8 hours at speeds of 64.5 kilometers per hour. They also logged the highest sustained climbing speed known from any bird species, of just under 1.1 vertical kilometers per hour. (Southbound geese do much less climbing because they start out high up on the Tibetan Plateau, so their trips took 4.5 hours or less.)

Most surprising was that the geese completed most of their journeys not during the day with the uplifting winds at their backs, but during the night or early morning, when headwinds were likely, according to data from a Mount Everest weather station. The researchers think the cooler night and early-morning temperatures, which would help dissipate body heat and increase oxygen availability, may be more helpful than any tailwind assist. The geese might also prefer calmer conditions for safety and maneuverability.

“These birds maintain an incredibly high oxygen consumption, one that might be maybe 10 times greater than resting, and they have to maintain that for ... hours on end,” Hawkes says. “It sets a new bar for aerobic exercise.” A few other bird species, including ducks and storks, also migrate over the Himalayas, but even less is known about their strategies and physiologies, Hawkes adds, so it’s uncertain whether they are in the same athletic class as the geese.

The study is “a nice confirmation that these geese are doing this climb by their own muscle power,” Anders Hedenström of Lund University in Sweden, an expert in animal aerodynamics and migration who has studied bar-headed geese, says via e-mail.

Physiologist Jessica Meir, a postdoctoral researcher at the University of British Columbia in Canada who studies flight in bar-headed geese, agrees, praising the new study for finally providing field data to lay to rest the entrenched notion that the geese take advantage of tailwinds, as smaller migratory birds have been shown to do. In fact, the geese’s strategy makes good sense, since loss of control could be deadly in the rugged Himalayan terrain, says Meir, who collaborates with some of the present study’s co-authors. “They showed in this paper that the birds were actually staying quite close to the mountain,” she says. “If there’s any change in the wind, then they might accidentally be hurled into the rock.”